1. Field of the Invention
The present invention relates to an image-forming apparatus which automatically detects the size of an original when the original is placed on an original table, and thereafter starts an image-forming process.
2. Description of the Related Art
In an image-forming apparatus, such as an electrophotographic apparatus, a size-detecting device is employed which detects the size of an original placed on an original table before the execution of a copying operation. The size-detecting device selects the sheets to be used for the copying operation or determines whether the size of the selected sheets is appropriate to the copying operation.
U.S. Pat. No. 4,963,934 discloses the size-detecting device to detect the size of an original, wherein the original table on which the original is placed is covered with a platen sheet, and the amount of light reflected by the original and the amount of light reflected by the platen sheet are sensed by sensors of the size-detecting device. The original and the platen sheet are discriminated from each other in accordance with the difference between the two amounts of light, and the size of the original is detected on the basis of the discrimination. Thereafter, the copying operation is started. As is understood from this, the original table on which the original is placed is first covered with the platen sheet, for the detection of the size of the original. Therefore, it takes time to finish the copying operation in the case where the size of the original has to be detected by the size-detecting device.
Moreover, there are many restrictions in determining a photosensitive material, the color of the platen sheet, the color of a light-emitting element, etc. For example, the platen sheet has to be colored for discriminating it from the original, and the color of the platen sheet has to be sensed with high sensitivity by the photosensitive member so as not to adversely affect the image produced in the copying operation. In addition, the light-emitting element must have a wavelength band in which the reflectance relative to the color of the platen sheet is low. In the case where an organic photoconductor is used as the photosensitive member, it is desirable that the color of the platen sheet be yellow, so as to ensure reliable image formation. It is also desirable that the color of the light-emitting element be blue since the blue color has low reflectance relative to the yellow color of the platen sheet. However, it is not easy to obtain a light-emitting diode which emits blue light, so that structural components are inevitably expensive, resulting in an increase in the manufacturing cost of the image-forming apparatus. As has been described, there have been both technical problems and cost problems in determining the photosensitive material, the color of the platen sheet, and the color of the light-emitting element.
According to a conventional image-forming apparatus, an original is placed on the original table in an original size detection mode, and the original size-detecting operation is started in synchronism with a copy start signal which is input by turning on the copy key of the apparatus. The light reflection type sensor stands by, together with an exposure device, at the predetermined reference position corresponding to the position of the rear end of a sheet of e.g. the A4 size. If the original placed on the original table is smaller than the A4 size, the sensor moves to the scan home position together with the exposure device when the copy key is turned on, and senses the rear end of the original during the movement. On the other hand, if the original is larger than the A4 size, the sensor moves away from the scan home position together with the exposure device so as to sense the rear end of the original. After the rear end of the original is sensed, the sensor and the exposure device move to the scan home position, and thereafter the copying operation is started.
As is understood from the above, if the size of an original is larger than a standard size (e.g., the A4 size), it requires a long time to produce the first copy in comparison with an ordinary copying operation which does not use the original size detection mode. Since the operator has to wait for a long time before the first copy is produced, the copying operation cannot be performed in a short time and with high efficiency.